Erythropoietin (EPO) is the glycoprotein hormone that controls red blood cell production and, thus, blood O2-carrying capacity. EPO deficiency is a major complication of chronic renal failure and parenteral EPO is utilized in the treatment of end-stage renal disease, AIDS, and cancer. EPO expression is primarily controlled at the level of gene transcription. Studies in cultured human Hep3B cells have revealed cis-acting DNA sequences and trans-acting factors that are required for transcriptional activation. The critical cis-acting element is a 50-base pair hypoxia-response element located in the 3'-flanking region of the EPO gene. The critical trans-acting factor is hypoxia-inducible factor 1 (HIF-1), a basic-helix-loop=helix-PAS protein. Expression of HIF-1 is induced in response to hypoxia and the duration and severity of the hypoxic stimulus determine the level of HIF-1 expression. Mutations that eliminate HIF-1 binding also destroy hypoxia response element function. The extent to which HIF-1 is required for endogenous EPO expression, either in cultured cells or in vivo has not been definitively established, although all data indicate that HIF-1 levels play a major role in determining the level of EPO gene transcription. The broad, long-term objective of gene expression as a function of O2 concentration. The specific aims are as follows: (1) To hypoxia-signal transduction pathway leading to induction of HIF-1 activity and EPO gene transcription. (3) To determine whether HIF-1 has a direct effect on erythropoiesis independent of endocrine EPO production and whether HIF-1 is required for autocrine EPO production by erythroid progenitor cells. Delineation of the molecular mechanisms by which EPO gene expression is activated may lead to the identification of low-molecular weight compounds capable of inducing EPO production in vivo. In addition, because HIF-1 is involved in the regulation of multiple genes that play important adaptive roles, our studies of EPO gene regulation will be applicable to other hypoxic/ischemic disease states, including myocardial ischemia, pulmonary hypertension, and stroke.